Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is particularly well suited for application of characters onto substrates such as glass, metal, or plastic containers and packages.
Reviews of various aspects of ink jet printing can be found in these publications: Kuhn et al., Scientific American, April, 1979, 162-178; and Keeling, Phys. Technol., 12(5), 196-303 (1981). Various ink jet apparatuses are described in the following U.S. Pat. Nos. 3,060,429, 3,298,030, 3,373,437, 3,416,153, and 3,673,601.
In general, an ink jet composition must meet certain rigid requirements to be useful in ink jet printing operations. These relate to viscosity, resistivity, solubility, compatibility of components and wettability of the substrate. Further, the ink must be quick-drying and smear resistant, resist abrasion, and be capable of passing through the ink jet nozzle without clogging, and permit rapid cleanup of the machine components with minimum effort.
Plastic substrates, particularly low surface energy plastics substrates such as polyolefins, present certain problems with respect to the quality and robustness of the printed message. The messages can be rubbed off or scratched off during normal handling with relatively moderate force if the adhesion of the message to the substrate is poor. In order to alleviate this problem, plastic surfaces are sometimes provided with a surface treatment such as oxidation, carona treatment or plasma treatment, or a coating of a primer. The above pretreatments add to the cost of the package manufacturing process. Thus, there is a need for a jet ink composition that avoids or minimizes the need for a pretreatment of a plastic substrate, particularly a polyolefin substrate.
Although ink jet ink compositions are known, ink jet ink compositions that are suitable for printing abrasion resistant messages on untreated, low energy plastic surfaces have been rare. The need for abrasion resistant ink jet ink compositions is clear from the patent and technical literature. For example, U.S. Pat. No. 5,594,044 discloses a jet ink composition suitable for printing messages having good adhesion and isopropyl alcohol rub resistance comprising an organic solvent, a flexible thermoplastic urethane resin, a colorant, a cellulose nitrate resin, and an adhesion promoter such as a titanate.
U.S. Pat. No. 4,207,577 relates to an ink jet ink composition that includes a cellulosic derivative, a resin component, and at least one solvent. Although the the patent states that the ink has adhesion to "practically all substrates", it appears that the patent is concerned with providing a jet ink for printing on virgin metal or coated substrates. See, e.g., column 3, lines 60-63: "depending upon [the] type of substrate on which ink is to be printed, it is advantageous to use a solvent which will penetrate the wax or other coating on substrates to enhance adhesion and rub resistance."
The foregoing indicates that there is a need for a jet ink composition that is suitable for printing on plastic surfaces, particularly low energy surfaces such as polyolefin surfaces, messages that have abrasion resistance in general, and in particular, messages that have rub resistance and fingernail scratch resistance.